The invention concerns an understructure for an automobile. The understructure being of a type with longitudinal members which support a drive unit and bear on sill members by way of cross members, and a floor panel forming a foot slope the course of which is followed by the longitudinal members so as to form gaps for vibration neutralization.
One problem in modern vehicle production with its tendency towards lightweight construction resides in keeping the interior noise level in the passenger compartment low, in spite of the utilization of body plates which are relatively thin.
The interior noise of the vehicle is determined essentially by low-frequency components which are excited by the drive unit with intake and exhaust systems as well as by the tire-road combination. At the internal combustion engine appear free forces and moments which cause the drive unit to vibrate. For example, in the widely utilized four-cylinder reciprocating-piston in-line engine are active forces of the second order of the engine speed in the direction of the piston movement, so that the entire unit undergoes vibrations. It vibrates at double the frequency of the engine speed in the piston movement direction and radiates, in particular above and below its contour, airborne sound which in part is resonance-like increased in the engine space and is introduced via the front wall and, respectively, the foot slope of the floor panel into the passenger compartment and, following reflection by the road surface, also penetrates the vehicle interior through the vehicle floor.
Due to the vibrations, there occurs in addition a solid-borne vibration excitation by way of the drive unit and undercarriage bearings as also further contact points, such as, e.g., hoses, clutch cable and steering assembly. Thereby flexural waves are introduced into the body so that numerous resonance-type plate vibrations occur which act like loudspeakers and together with the aforedescribed airborne sound component produce the low-frequency interior noise. Frequently, air space modes of the vehicle interior are excited, too, which leads to particularly troublesome humming increases.
However, the vehicle interior noise is not generated everywhere at the same level. In some body regions, very small body vibrations are sufficient to produce disagreeable humming increases. These sites are, e.g., the front wall, bottom, the floor slope, front, and the rear leg space ahead of the rear seat as well as the front roof region. The sounds radiated here can interfere with one another because of the spatial dimensions, e.g., between the front and rear leg spaces, on the one hand, (sources), and the ears of the vehicle occupant, on the other hand, (receivers), and as a result, interior noise amplifications as well as interior noise decreases can be caused according to undesirable interior noise dynamics over extended speed ranges.
In the understructure for automobiles known from characteristics DE-OS No. 21 45 467, B62D 23/00 which contains the characteristics of the above-mentioned type of understructure, the longitudinal members are completely uncoupled from the floor panel by gaps. Force introduction is effected directly into the sill members, namely, by way of torsion boxes into the front end regions of the sill members extending below the A columns as well as by way of individual cross members into sill member regions behind the former. It is true that this uncoupling has a favorable effect as regards a direct vibration excitation of the floor panel originating with the drive unit. However, such a construction does not take into account the vibration excitation due to reflection by the road surface described above.